.box{
    width: 100%;
    height: 126px;
    border: 1px solid red;
    position: relative;
}
.textarea{
    width: 100%;
    height: 100%;
    border: none;
    outline: none;
    resize: none;
}
.btn{
    position: absolute;
    right: 10px;
    bottom: 10px;
}